qemu: Support mountpoints option of guest-fsfreeze-freeze

qemu: Support mountpoints option of guest-fsfreeze-freeze

With this patch, virDomainFSFreeze will pass the mountpoints argument
to qemu guest agent. For example,

  virDomainFSFreeze(dom, {"/mnt/vol1", "/mnt/vol2"}, 2, 0)

will issue qemu guest agent command:

  {"execute":"guest-fsfreeze-freeze",
   "arguments":{"mountpoints":["/mnt/vol1","/mnt/vol2"]}}

Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Acked-by: Daniel P. Berrange <berrange@redhat.com>